1. Field of the Invention
The invention relates to communications systems and more specifically to communications systems utilizing a "lines" transponder and the intermodulation products generated by the transponder to produce a control signal for controlling the output power of the transmitter such that the power level of the input signal to the transponder is maintained at the desired level.
2. Description of the Prior Art
Many prior art transponder systems, such as satellite, have utilized carrier frequencies less than 10 gigahertz. These frequencies are not subject to severe fading due to rain showers, for example. Recent developments have indicated the desirability of operating some of the systems in the 10 gigahertz and above frequency range. At these frequencies changes in the attenuation of the signal between the transmitter and the transponder can result in significant variations in the power level of the input signal to the transponder. If the transponder is to operate in the "linear" mode, that is the power level of the output signal or the transponder has a substantially direct relationship to the power level of the input signal, some system for determining the power level of the input signal to the transponder is required in order to maintain the power level of the output signal of the transponder at the desired level. Additionally, transponders operating in the linear mode such as the CTS satellite may simultaneously handle input signals from different sources. In this situation, it becomes necessary to independently determine the power of each of the input signals as they arrive at the transponder.
Conceptually, instrumentation could be included in the transponder to independently measure the power of each of the input signals as they arrive at the transponder. Obviously this can lead to considerable complexity. In fact, it may even become technically impractical in transponders simultaneously handling a large number of input signals.